(1) Field of the Invention
The invention relates to the manufacture of highly dense integrated circuits and more particularly to the formation of a retarded double diffused drain structure in a field effect transistor.
(2) Description of the Related Art
In the manufacture of highly dense integrated circuits using Metal Oxide Semiconductor Field Effect Transistors (MOSFET) technology, as device dimensions decrease, there has been a need to create shallower source/drain regions. However, corresponding high doping concentrations lead to an increase in the electric field in the device channel in the region adjacent to the drain. This high electric field causes electrons in the device channel to gain energy and be injected into the gate oxide. This phenomenon is known as the "hot electron" problem. This in turn leads to long-term device degradation and reduced reliability.
A method to reduce the high electric field at the drain region is to provide a more gradual, or graded, change in the doping concentration at the drain/channel interface. One means of accomplishing this is with a lightly doped drain (LDD), as described in "VLSI TECHNOLOGY", by S. M. Sze, published by McGraw-Hill International -Singapore, 1988, pages 482-483. As shown in FIG. 1, a substrate 10 is implanted with a heavy implant to create regions 12, and an implant with lighter doping concentration to form regions 14. A smaller electric field results at the drain/channel interface than would exist in a device in which a single heavy implant was performed, due to a reduction in the difference in dopant concentrations between the channel and the drain region adjacent to it.
A second approach to minimize the hot electron problem is shown in U.S. Pat. No. 4,851,360 by Haken et al. In this "double diffused drain" method, two implants are performed using the same mask, and results in the FIG. 2 structure. A first implant of phosphorus is followed by a second implant of arsenic. The greater diffusivity of phosphorus causes it to diffuse laterally during the implant anneal, to form region 16, which has a lighter resultant dopant concentration than adjacent region 18 formed by the arsenic implant.
A drawback to the double diffused drain approach is an increase in source/drain junction capacitance. This leads to an increase in the RC constant and an undesirably higher propagation delay, which results in slower performance of the circuits formed with these devices.